Antimony plating process



United States ANTIMONY PLAHNG PROCESS No Drawing. Application August 17,1956 Serial No. 604,593

6 Claims. (Cl. 204-49) This invention relates to a process for thedeposition of metallic coatings on metal surfaces. More particularly, itrelates to the electrodeposition of adherent and continuous coatings ofantimony on steel.

It is an object of this invention to deposit antimony coatings on steeldirectly, without an intermediate coating of another metal. Anotherobject is to deposit the coating of antimony directly on the steelwithout roughening the surface of the steel previous to the coating, andthereby to produce a smooth coating. A further object is to apply anantimony coating on low-carbon steel continuously and completely overthe steel surface. A still further object is to provide a method ofcoating steel directly from weak antimony acid plating baths.

With the exception of copending patent application, Serial No. 604,630,insofar as is known, no entirely satisfactory method of coating antimonyon steel has been found. While certain patents in the prior art alludeto successful methods of coating with antimony directly, these methodsrequire the utilization of strongly corrosive antimony fluoride acidsolutions and are therefore not completely satisfactory. Patent2,683,114 is representative of the prior art in this respect. When theseprior art methods are used to plate antimony directly on steel, themethod calls for toughening the steel by sandblasting or etchingprocesses.

Other prior art methods also disclose the more common method of antimonyplating, in which a coating of another metal, such as copper, zinc, orother nonferrous metal, is coated on the steel object prior to theantimony plating operation.

In the process of this invention, the antimony plating may be carriedout in a weak citric or other aliphatic hydroxy carboxylic acid bathdirectly on an unroughened and polished steel object because of thetreatment previously given to the surface of the object. The antimonyplate produced by the method of this invention is strongly adherent andmay be easily buffed to a high polish. in addition, the antimony plateis continuous, covering the exposed surface completely without leavingunplated spots.

Briefly, the process of this invention includes a new process stepimmediately prior to the antimony plating step. This new process step isthe anodic treatment of the object to be coated in an acid solution fora short time at a current density sutficient to cause polarization atthe anode object and passivation with the consequent liberation of gastherefrom. The anodic acid treatment step is immediately followed with arinse step and the antimony plating operation.

While the reasons for the success of this coating process are notdefinitely known, it is believed that the anodic acid treatment stepleaves a chemically uniform surface over all areas of the object and thesuccessful continuous adherence of antimony directly on steel requiressuch a chemically uniform surface. Objects on which a coating ofantimony has" been applied directly without the benefit of the newanodic acid treatment have been found to atent O have many dispersed,randomly positioned, unplated spots on the surface. In theelectroplating operation, it has been observed that those areas whichwill receive antimony plate will commence to plate in the very first fewseconds after the current is turned on. However, those nonreceptivespots Will be immediately apparent, and continued time in the platingoperation will not cause any deposit on these spots. It is believed thatthe reason for the nonreceptive spots may be the existence at thesepositions of a greater overvoltage for the antimony than theo-vervoltage for hydrogen in the plating bath. It is thought that theprovision of the anodic acid treatment previous to the plating operationreduces the overvoltage of the antimony at these spots to a value lessthan the overvoltage for hydrogen in the plating bath and that achemically uniform surface results, making possible the continuous,over-all antimony plating.

In the operation of the method of this invention, certain mineral acidshave proved to be effective acid solutions. Sulfuric, phosphoric, andhydrochloric acid solutions have all been used.

A mixture of about 24 to 62 percent. by weight sulfuric acid with waterhas been found to give good direct antimony plating results.

A mixture of about 53 to 85 percent by weight phosphoric acid with waterhas also been found to give good results.

In addition, a mixture of about 26 to 37 percent by weight hydrochloricacid with water has been found to give good results.

The operating temperature and concentration of the treating solutiondetermine the current density required. In the process, the currentdensity must be high enough to cause polarization of the anode objectand passivation with the consequent liberation of gas from the object.At normal room temperatures, successful treatments have been carried outat a current density of about amperes per square foot with solutions inthe higher concentration range, for instance, 62 percent for sulfuricacid; percent for phosphoric acid; and 37 percent for hydrochloric acid.At the lower concentrations, and at room temperature, successfultreatments have been made at a current density of amperes per squarefoot.

The time required for the anodic acid treatment is preferably about twominutes. At the usual current densities, bath temperature, and acidconcentrations, described above, between 1 and 30 seconds is required toproduce polarization at the anode object. Passivation with theconsequent liberation of gas, probably oxygen, follows shortlythereafter. The process must be carried on for a sufiicient length oftime to cause gas liberation. Once the passivation occurs at the anodeobject, the process may be stopped. However, it is preferred that theprocess be continued for up to a total of 2 minutes. The process may becontinued even longer, and a period of up to 5 minutes may be used toinsure treatment over all surfaces of an intricately shaped object.Treatment of longer than 5 minutes appears to be wasted, as no furthereffect has been found after 5 minutes.

While the exact nature of the surface of the object just previous to,during, and immediately after the comrnencement of gas liberation is notknown, it is believed that the surface may be pure iron and that thetreatment may remove most other impurities from the surface and leave alayer one atom thick of iron. The liberation of gas, which is probablyoxygen, may leave this surface with an absorbed layer of oxygen on it.It has been found that, after the passivation of the steel objectoccurs, substantially no iron dissolves. Thus, the physical condition ofthe surface of the object, such as the roughness thereof, is notdisturbed, for all practical purposes, by the anodic acid treatmentprocess of this invention.

Example I A typical plating process of 4 x 6-inch flat steel panels, inwhich the anodic acid treatment is combined, is as follows:

(1) The object to be coated is cleaned in a conventional alkalinecleaner solution.

(2) The object is rinsed in water.

(3) The object is immersed or dipped in an acid bath to remove oxide,smut, and other surface impurities. Either sulfuric or hydrochloric acidmay be used.

(4) The object is rinsed in water.

, (5) The object is connected as the anode in an acid solution of about53 percent by weight H PO The treat- :ment is conducted for two minuteswith the acid at a .4 venience, and an economic factor in the favor ofroomtemperature operation.

An additional advantageto be found in the process of the invention isthe use of relatively dilute acids. The use of dilute acids results inthere being less effect on the strength of the treating bath by watertransmittal from the cleaning solution and the rinsing bath. In anyplating operation, there is the problem of carry-over from one solutionto the next in the successive dipping operations. In addition, the useof dilute acids provides better control over the operation, in that thetreating solution, if sulfuric acid is used, does not change itsstrength hecause of hygroscopic absorption from the air;

temperature of between 70 and 90 F. at a current density of about 100amperes per square foot.

(6) The object is rinsed in water.

(7) The object is electroplated in a citric acid bath of the followingcomposition and under the following operating conditions: m Composition:

Potassium citrate (K C H O -H O) g./l 145 Citric acid (H3C5H507) g./lAntimony oxide (Sb O .g./l 60 Water, to make one liter. pH 3.5-3.7

Operating conditions:

Temperature F 120 Anodes Antimony Cathode current density amp./sq. ft 30Example II A typical plating process on an intricately shaped steelobject, in which the anodic acid treatment step is combined, is asfollows:

(1) The object to be plated is cleaned in a conventional alkalinecleaner solution.

' (2) The object is rinsed in water.

(3) The object is immersed or dipped in an acid bath to remove oxide,smut, and other surface impurities.

(4) The object is rinsed in water.

(5) The object is connected as the anode in an acid solution of 62percent by weight H 50 with water. The treatment is conducted for 5minutes with the acid at a temperature of between 70 and 90 F. at acurrent density of about 75 amperes per square foot.

(6) The object is rinsed in water.

(7) The object is electroplated in a citric acid bath of the compositionand under the operating conditions described in Example I.

, It has been found that mixtures of the three abovedescribed mineralacids may be used if the strength of the individual acid in the solutiondoes not exceed the maximums set forth herein. The ability to mix theacids provides further flexibility in the operation of the process.

Normal brightening agents conventionally used in plating baths are notaffected by the anodic treatment of this invention.

One of the advantages to be found in the process of 'this invention isin the successful treatment at normal room temperatures. It is apparentthat any process operation which may be carried out at normal ambienttemperatures is better than similar processes which must be operated atelevated temperature because there is no need for temperature controland heat-addition or subtraction apparatus. Of course, continued use ofthe bath may raise the bath temperature because of the resistance of thesolution to current passage. This increase in temperature has nodetrimental effect on the operation, ex.

cept to require higher current densities. However, the .labthty tooperate initially at room temperature is a conwith flexibility that isadvantageous.

In view of the fact that the three separate acidssulfuric, phosphoric,and hydrochlorichave been found to give good treating results inpreparing a steel object for antimony plating, the plating operation isprovided The particular treating solution may be selected on the basisof its compatibility with the plating bath. Therefore, any carryoverfrom the treating solution through the rinsing solution to the platingsolution by the successive dipping process will not harm the platingbath.

In-the prior art method of coating antimony on steel by means ofstrongly corrosive antimony fluoride acid baths, the corrosive nature ofthe plating bath is a decided disadvantage. The use of such highlycorrosive acids makes necessary the provision of expensive ventilatingand handling equipment. This invention, by making possible the use ofweak aliphatic hydroxy carboxylic acids eliminates these problems.

In the prior art methods of plating antimony directly on steel by strongantimony fluoride acid solutions, there is also the disadvantage thatthese acid solutions attack the object to be plated during the earlystages of the plating operation before the object is covered withantimony plate, and, therefore, there is a loss of metal from the objectand contamination of the plating bath.

In the prior art methods of coating antimony on steel by means ofintermediate coatings, it has been found that intermediate coatings,such as copper and zinc, form alloys with antimony, by room-temperaturediffusion, and that these alloys are brittle and cause the platedcoatings to peel. Direct coatings by the method of this invention havebeen formed to an angle of degrees after being stored for six months,and the coatings have been found to be pliable with no tendency to peel.

It has been found that electropolishing will also condition the surfaceof a steel object to accept antimony plating. However, theelectropolishing operation removes metal but the anodic treatmentprocess does not. Of course, in a process which removes metal, there isthe disadvantage of bath contamination caused by the metal which issuccessively removed from the succeeding objects; in addition, when thecontamination builds up in the electropolishing bath, the iron saltswill often be carried into the antimony plating bath in a normal,successive-dipping cycle.

It will be understood, of course, that while the forms of the inventionherein shown and described constitute preferred embodiments of theinvention, it is not intended herein to illustrate all of the possibleequivalent forms or ramifications of the invention. It will also beunderstood that the words used are words of description rather than oflimitation and that various changes may be substituted without departingfrom the spirit or scope of the invention herein disclosed.

What is claimed is:

1. The process of preparing a steel object to receive anelectrodeposited coating of antimony comprising: preceding theapplication of said antimony on said steel object by treating said steelauodically in an acid solution at a current density of at least about 75amperes per square foot for a sufiicient length of time to causepassivation at said anodic object with consequent liberation of gastherefrom, the physical condition of the surface of said object notbeing disturbed thereby; said acid being selected from the groupconsisting of sulfuric acid, phosphoric acid, and hydrochloric acid.

2. In the process of plating antimony on a steel object comprisingcleaning said steel object followed by a first rinsing operation, aciddipping said steel object followed by a second rinsing operation, andelectroplating said steel object with antimony; the step following saidsecond rinsing operation of treating said steel object anodically in anacid solution, at normal room temperature at a current density of fromabout 75 to 100 amperes per square foot for sufficient length of time tocause passivation at said anodic object with consequent liberation ofgas therefrom, the physical condition of the surface of said object notbeing disturbed thereby; said acid soution being selected from the groupconsisting of sulfuric acid, phosphoric acid, and hydrochloric acid.

3. The process of electrodepositing antimony on a steel objectcomprising: cleaning said steel object in an alkaline cleaning solution;rinsing said steel object; treating said steel object in an acid dip;subjecting said steel to a second rinse; treating said steel objectanodically in an acid solution, wherein said acid is selected from thegroup consisting of sulfuric acid, phosphoric acid, and hydrochloricacid, for a period of between 1 and 300 seconds at normal roomtemperature, and at a current density of from 75 to 100 amperes persquare foot, said period being a sufficient length of time to causepassivation at said anodic object with consequent liberation of gastherefrom, the physical condition of the surface of said object notbeing disturbed thereby; subjecting said steel to a third rinse; andelectroplating antimony on said steel in an aliphatic hydroxy carboxylicacid plating solution.

4. A process of preparing steel for the electrodeposition of antimonycoatings thereon comprising: preceding the application of said antimonyon said steel by treating said steel anodically in a sulfuric acidsolution of about 62 percent by weight at a temperature of between 70and 90 F. for a period of between 1 and 300 seconds at a current densityof about 75 amperes per square foot, said anodic treatment causingpassivation at said anodic object with consequent liberation of gastherefrom, the physical condition of the surface of said object notbeing disturbed thereby.

5. A process of preparing a steel object for the electrodeposition ofantimony coatings thereon comprising: preceding the application of saidantimony on said steel object by treating said steel object anodicallyin a solution of about 85 percent by weight phosphoric acid at atemperature of between and 90 F. for a period of between 1 and 300seconds at a current density of about amperes per square foot, saidanodic treatment causing passivation at said anodic object withconsequent liberation of gas therefrom, the physical condition of saidobject not being disturbed thereby.

6. A process of preparing a steel object for the electrodeposition ofantimony coatings thereon comprising: preceding the application of saidantimony on said steel object by treating said steel object anodicallyin a solution of about 37 percent by weight hydrochloric acid at atemperature of between 70 and F. for a period of between 1 and 300seconds at a current density of about 75 amperes per square foot, saidanodic treatment causing passivation at said anodic object withconsequent liberation of gas therefrom, the physical condition of thesurface of said object not being disturbed thereby.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Principles of Electroplating and Electroforming, 3rd edition,Blum and Hogaboom, McGraw-Hill Book Company, Inc., New York, 1949.

Modern Electroplating, edited by A. G. Gray, John Wiley & Sons Inc., NewYork, 1953, pp. 486 and 487,

1. THE PROCESS OF PREPARING A STEEL OBJECT TO RECEIVE ANELECTRODEPOSITED COATING OF ANTIMONY COMPRISING: PRECEDING THEAPPLICATION OF SAID ANTIMONY ON SAID STEEL OBJECT BY TREATING SAID STEELANODICALLY IN AN ACID SOLUTION AT A CURRENT DENSITY OF AT LEAST ABOUT 75APMERES PER SQUARE FOOT FOR A SUFFICIENT LENGTH OF TIME TO CAUSEPASSIVATION AT SAID ANODIC OBJECT WITH CONSEQUENT LIBERATION OF GASTHEREFROM, THE PHYSICAL CONDITION OF THE SURFACE OF SAID OBJECT NOTBEING DISTURBED THEREBY; SAID ACID BEING SLECTED FROM THE GROUPCONSISTING OF SULFURIC ACID, PHOSPHORIC ACID, AND HYDROCHLORIC ACID.